linux-kernel - Re: slow nanosleep?

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Message-ID: <1283950818.23762.20.camel@laptop>
Date:	Wed, 08 Sep 2010 15:00:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: slow nanosleep?

On Wed, 2010-09-08 at 14:43 +0200, Thomas Gleixner wrote:
> > However nanosleep with 1 ns and prctl(PR_SET_TIMERSLACK, 1) takes
> > about 8 us on x86(Intel(R) Core(TM)2 Duo CPU E8500  @ 3.16GHz)
> > and 20 us on my slower ppc board. Is that system call overhead
> > or possibly some error?
> 
> That's overhead I fear. We go way up to enqueue/arm the timer until we
> figure out that the timeout already happened. 

Well, there's also the fact that his ppc board is simply dead slow,
using the freq ratio: 3166/266 you'd expect (at a similar ins/clock
ratio) the ppc to take 95us.

So in fact the pcc taking 20us is actually quite good.


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